It is the purpose of this site to provide a deep, non-technical
review of stars and their natures for the beginner. This page
presents facts about stars as we know them without delving into the
details of discovery. A parallel site, now under construction,
will explore the spectra of the stars and will examine how we have
learned so much of what is presented here. The two sites will be
linked, allowing you to go back and forth between them to see how
stars are born, live their lives, and die, in the process creating
other stars, perhaps other earths, and all that is around us.
In the second century BC, the Greek astronomer Hipparchus divided
the stars into six brightness groups called magnitudes, first
magnitude the brightest, sixth the faintest. The system is still
used today, though with a mathematical definition (a star of one
magnitude is 2.5 times brighter than the next fainter) that takes
the very brightest stars and planets through magnitude zero and
into negative numbers. Through the telescope we see much fainter,
to near 30th magnitude (4 billion times fainter than the human eye
can see alone). Though stars bear some resemblance to the Sun,
they appear as points in the sky because they are so far away, the
nearest, Alpha Centauri, four light years away. The light year is
the distance a ray of light will travel in a year at 300,000
kilometers per second, so one light year is about 10 trillion
kilometers (63,000 times the distance between the Earth and the
Sun). The stars are so far that distances were not measured until
1846, by means of parallax (viewing the star from opposite sides of
the Earth's orbit). The most distant stars the unaided eye can see
are over 1000 light years away, which is about the practical limit
of parallax measures.
A star is a body that at some time in its life generates its
light and heat by nuclear reactions, specifically by the fusion of
hydrogen into helium under conditions of enormous temperature and
density. When hydrogen atoms merge to create the next heavier
element, helium, mass is lost, the mass (M) converted to energy (E)
through Einstein's famous equation E = mc squared, where "c" is the
speed of light. The Sun is powered by hydrogen fusion, as are
many of the other stars you see at night. The fusion does not take
place throughout the star, but only in its deep interior, in its
core, where it is hot enough. The temperature at the center of the
Sun is 15 million degrees Kelvin (K = centigrade degrees above
absolute zero, -273 C), and the density is 10 times that of lead.
About 40% of the mass of the Sun, occupying about 30% of the
radius, is capable of fusing hydrogen. Even under these extreme
conditions, the Sun is still a gas throughout.
To create the conditions for such "thermonuclear fusion," stars
must be massive. The Sun has the mass of 333,000 Earths. Stars
can range up to about 100 times the mass of the Sun (at which point
nature stops making them) down to around 8% that of the Sun, at
which point the internal temperature is not high enough to run the
full range of nuclear reactions (which requires at least 7 million
degrees Kelvin). "Substars" below the 8% limit, called "brown
dwarfs," do exist however.
Stars are made of the same chemical elements as found in the Earth, though not in the same proportions. Most stars are made almost entirely of hydrogen (about 90% by number of atoms) and helium (about 10%), elements that are relatively rare on our planet. About a tenth of a percent is left over, that tenth containing all the other elements found in nature. Of these, oxygen usually dominates, followed by carbon, neon, and nitrogen. Of the metals, iron usually dominates. Nevertheless, there is only one atom of oxygen in the Sun for every 1200 hydrogen atoms and only one of iron for every 32 oxygen atoms. However, within this tenth of a percent, the proportions of the numbers of atoms in the Sun is rather similar to what we find here. Other stars can deviate considerably, depending on their states of aging.
The space between the stars is filled with dusty gas. Thick dust clouds can even be seen with the naked eye
within the Milky Way blocking the light of distant stars and
providing much of the Milky Way's structure. These clouds can be
compressed through collisions or by blast waves from exploding
high-mass stars. Lumps of matter therefore form within the
interstellar clouds. If their gravity is great enough, they can
condense into one or more stars. The contraction of forming stars
raises the internal temperature, finally to the point of ignition
of hydrogen fusion. Gravity would like to make the star as small
as possible, but the fusion reactions stabilize it and keep it from
contracting any further. The whole life story of a star from here
on out is told by the battle between gravity and nuclear fusion,
first one, then the other getting the upper hand.
As a new star condenses from a gaseous lump in interstellar space, it spins faster, the outer parts of the contracting cloud spinning out into a dusty disk. The dust particles, in orbit about the new star, accumulate, building themselves into planets. Here at home, the planets that formed close to the Sun (Mercury through Mars) were in an environment too hot to incorporate much water of light atoms like hydrogen, so they are made of heavy stuff like iron, silicon, and oxygen. In the outer System, the planets contain huge amounts of hydrogen and helium and could grow large, their satellites made largely of water ice. Other stars should grow planets too, planets that could be quite different from our own and that are now being discovered.
There are many kinds of stars. Those that are actively fusing
hydrogen into helium in the middle, that is, in their cores, are
called "main sequence" stars. The main sequence is the first stage
following birth. In general, main sequence stars have chemical
compositions similar to that of the Sun. The higher the mass of
the main sequence star, the greater its diameter and the higher its
surface temperature. Dimensions range from about 5% the size of
the Sun (which is 1.5 million kilometers -- 109 Earths -- across)
to about ten times solar, and surface temperatures from about 3000
degrees Kelvin to about 50,000 K (the Sun's surface is at 5800 K).
Around the beginning of the 20th century, astronomers divided the
stars into seven basic lettered groups that they later learned were
related to surface temperature, O (above 30,000 K), B (9500 -
30,000 K), A (7000 - 9500 K), F(6000 -7000 K), G(5200 - 6000 K),
K(3900 - 5200 K), and M (below 3900 K). The Sun is a G star. The
system is decimalized, making the Sun class G2. Examples of naked-
eye main sequence stars are Vega, Altair, and Sirius.
These classes are actually derived from the stars' spectra. Since the color of a heated body
depends on temperature, the different classes take on different,
though subtle, colors, from slightly reddish for class M to orange
for K, through yellow-white to bluish for classes B and O. Star
colors can be noted rather easily even with the unaided
eye.
Main sequence stars have only a certain amount of internal fuel
available within their hot cores. When the hydrogen fuel has all
turned to helium, the stars begin to die and to produce a number of
other different kinds. Because higher mass stars use their
hydrogen fuel much more quickly than lower mass stars, those of
higher mass live shorter lives. The Sun has a 10 billion year main
sequence lifetime (of which half is gone). The most massive stars
live only a million years, the least massive for trillions, so long
that no star with a mass less than 0.8 solar masses has ever died
in the history of the Galaxy (our home collection of 200 billion
stars), which is about 15 billion years old.
When the fuel in a star's core runs out, the helium core contracts under the effect of gravity and heats up. Hydrogen fusion then expands into a shell around the old burnt-out core, and so much energy is produced that the star temporarily brightens and expands by many times over, the expansion cooling the surface, turning the star into a class M "red giant." When the temperature hits around 100 million degrees Kelvin, the helium is hot enough to fuse into carbon and even a bit further, into oxygen. This new power source stops the core's contraction and the star stabilizes for a time, dimming and heating somewhat at the surface. We commonly see these helium-fusing stars as type K giants. Good examples are Aldebaran and Arcturus. Such stars have diameters tens of times that of the Sun. The giant and subsequent stages up to the actual death of the star -- the end of nuclear fusion -- takes roughly 10% of the main sequence lifetime.
When the helium in the core has turned to carbon and oxygen, the
core shrinks again, and the helium begins to fuse to carbon and
oxygen in a shell around the old core, this shell surrounded by
another one fusing hydrogen into helium, the two turning on and off
in sequence. The star now brightens again, expands even more, and
becomes cooler and even redder than before. As the star brightens
it becomes unstable and begins to pulsate, the pulsations making it
vary, or change in brightness. The star become so huge, near or
greater than the orbit of the Earth, that the pulsations can take
a year or more. The first of these found, Mira in Cetus, changes
from second or third magnitude to tenth, becoming quite invisible
to the naked eye. Such stars are now called "long-period" or "Mira
variables." Thousands, all cool class M giants, are
known.
The gases of red giants can circulate upward to the tops of the stars, carrying the by-products of nuclear fusion with them. Oxygen is normally more abundant than carbon. If conditions are right, the surfaces of some stars can change their chemical compositions, some becoming very rich in the carbon that was made below by helium fusion, resulting in the reversal of the normal ratio. Mira variables and other old red giants thus divide into oxygen-rich stars and "carbon stars." Raised up along with the carbon are elements such as zirconium and many others that have been made in a huge variety of nuclear reactions that go on at the same time as helium fusion. Other stars' surfaces are enriched in helium and nitrogen.
Such huge giant stars have low gravities and lose mass through powerful winds that blow from their surfaces. Some of the gas condenses into molecules and dust. There may be so much that the star can be buried in it and become invisible to the eye, the glow of the heated dust seen only by its infrared (heat) radiation. Oxygen-rich giant stars make silicate dust, while carbon stars make carbon-dust similar to graphite and soot. Most of the dust that inhabits interstellar space began this way, though since inception it has been highly modified in the freezer of interstellar space. These stars therefore play a powerful role in later star formation. The winds are so strong during the giant stage of a star's life that it can lose half or more of its mass back into space, whittling itself down to little more than the parts that underwent nuclear fusion.
As a giant star loses almost all of its remaining outer hydrogen envelope, it comes close to revealing its intensely hot core. A fast wind from the core first compresses the inner edge of the old expanding wind. High-energy radiation from the hot core then lights up this inner compressed portion, which is now many times the size of the whole Solar System. These illuminated clouds, which can be quite beautiful, were discovered by William Herschel around 1790, who termed them"planetary nebulae" for their disk-like appearances (they have nothing else to do with planets). Their complex appearances depend to a degree on how matter is lost from the giant stars that make them. Expanding at rates of tens of kilometers per second, they last no more than a few tens of thousands of years.
As the planetary nebula dissipates into the gases of interstellar
space, it leaves behind the spent, old core (that now includes the
dead nuclear fusing shells). These stars, compressed under their
gravity, have shrunk to only about the size of Earth. The first
ones found were fairly hot and white, so the class acquired the
name "white dwarf" to discriminate it from the main sequence of
stars (which were originally called "ordinary dwarfs" to
distinguish them from the giants). Though small, white dwarfs
still contain near the mass of the Sun, giving them astonishing
average densities of a metric ton per cubic centimeter. The
tremendous outward pressure exerted under the great density
prevents gravity from shrinking them any further. White dwarfs, no
longer having any source of energy generation, are destined only to
cool. The cooling time is so long, however, that all white dwarfs
ever created are still visible, though the oldest are becoming
cool, dim, and reddish. There is no such thing a "black dwarf."
Higher mass stars, those with masses over about 10 times that of
the Sun, develop the same way as giants as they start to die, but
then their course of evolution becomes very different. High mass
stars are already large and luminous. As their dead helium cores
contract, heating and firing to fuse the helium to carbon and
oxygen, the stars expand to approach the sizes of the orbits of the
outer planets, becoming distended red "supergiants." Excellent
examples are first magnitude Betelgeuse in Orion and Antares in Scorpius. Supergiants are so
massive, in spite of great mass loss through huge winds, that
nuclear fusion can proceed farther than it can in ordinary giants.
When the helium runs out, the carbon and oxygen mixture compresses
and heats, causing it to fuse to a mixture of neon, magnesium and
oxygen. Hydrogen and helium fusion had already moved outward into
nested shells around the core. When carbon fusion dies out in the
core, leaving a mix of neon, magnesium, and oxygen, it too moves
outward into a shell. The neon-magnesium-oxygen mixture now in the
core then heats and fuses into a mix of silicon and sulfur, each
fusion stage taking a shorter period of time. During the course of
their evolution, red supergiants can also contract some and heat to
make blue supergiants. The great mass-loss suffered by supergiants
can strip some of them of their outer envelopes to the point that
we see huge surface enrichments of helium, nitrogen, and carbon
that have been made by nuclear fusion.
Finally, the silicon and sulfur fuse to iron, an element that is
incapable of energy-generating fusion reactions. Gravity now wins
the war that has been going on for the star's lifetime, and since
the iron refuses to support itself, the core catastrophically
collapses. The iron breaks down into its component particles,
protons, neutrons, and electrons (the constituents of atoms), and
the whole mass gets compressed into a tight ball of neutrons only
a few tens of kilometers across. The collapse produces a shocking
blast wave that rips through the surrounding nuclear fusing shells
and the remaining outer envelope, and rips the rest of the star
apart. On Earth we see the star explode in a grand "
supernova," an event so powerful it is easily visible even in
another galaxy a huge distance away.
There are ways of making supernovae other than through core
collapse. Nevertheless, supernovae are still rare, taking place in
our Galaxy only two or three times a century. Most are hidden from
us by the vast clouds of dust that birth the stars. On Earth we
observe about five supernovae per millennium, and have not seen one
since Kepler's Star of 1604 (probably created in the collapse of a
white dwarf, as described later), which was so bright that it was
visible in daylight. Our knowledge of supernovae comes almost
entirely from observing them in other galaxies, the best of these
exploding in 1987 in the Large Magellanic Cloud, a companion to our
Galaxy some 170,000 light years away. But keep your eye on
Betelgeuse or Antares, which are quite good candidates for core
collapse. An even better candidate is the southern hemisphere's Eta
Carinae, which should go within the next million years or so.
At their current distances, the explosions of such stars would
rival the brightness of a crescent Moon. The blast is so powerful
that it if occurred within 30 or so light years, it would probably
damage the Earth. Fortunately, no candidate is nearly that
close
As the debris of a supernova clears, we see a gaseous expanding
shell around the old star, the "supernova remnant," the debris rich
in the by-products of myriad nuclear reactions. We believe all the
iron in the Universe has come from such (and related) explosions.
Indeed, between ordinary giants, planetary nebulae, and supernovae,
all the elements other than hydrogen and helium were created in
stars. The most famous supernova remnant is the Crab
Nebula in Taurus, the remains of the great supernova of 1054,
which was well observed by Chinese astronomers. Tens of thousands
of years after the explosion we can still see the mighty blast
waves sweeping through the gases of interstellar space, compressing
them and perhaps making new stars.
At the center of the expanding cloud is a lone neutron star
spinning many times per second, with a mass greater than the Sun,
a diameter the size of a small town, and an amazing density of 100
million tons per cubic centimeter.The magnetic fields of such
collapsed stars are magnified along with the density to strengths
millions of millions of times that of Earth. The magnetism is so
strong that radiation is beamed out the magnetic axis. The axis is
tilted relative to the rotation axis (like that of the Earth), and
wobbles around as the little star spins, the beamed energy spraying
into space. From a distance, the star looks like a lighthouse: if
the Earth is in the way, we get a blast of radiation, and from here
see the neutron star as a "pulsar."
Young pulsars emit from low-energy radio waves through high-energy
X-rays and gamma rays. As the pulsar ages, it slows, and finally
emits only radio waves, which is the case for most of the 600 or so
pulsars known. When the rotation period is about 4 seconds there
is insufficient energy for the pulsar to be seen at all, and it
disappears from view. Not fusing anything, the neutron star is
held up forever against gravity by pressure exerted its own extreme
density.
The collapsing star of a supernova will turn into a neutron star
only if its mass is less than about two or three times that of the
Sun. If the mass is greater, then even the star's huge density
cannot hold gravity back, and instead of a neutron star the
supernova creates a "star" that nothing can support against
gravity, and the body contracts forever. At a small enough radius,
the gravitational force becomes so great that light can escape, and
the star disappears forever into a collapsing "black
hole." What we refer to as the black hole is actually a kind
of "surface" at which the velocity required for escape equals
light-speed. What goes on inside is unknown.
Most of stars you see at night have companions, a great many obviously double even through a modest telescope. The components of some double stars are nearly equal in mass and brightness. More commonly, one dominates the other, sometimes to the point where a little companion is not really visible at all, and detectable only with the most sophisticated techniques. At the lowest end, we have stars with low-mass brown dwarfs for companions. The stars of some doubles are so far apart that they take thousands of years to orbit; others are so close that they revolve around each other in only days or even hours. Gravitational theory allows us to measure the masses of the stars from the orbits' characters; indeed such measurements are the only way in which we can find stellar masses.
When a new star condenses from the interstellar gases, it spins faster. If the contracting blob is spinning rapidly enough, it can separate or otherwise develop into a pair or stars rather than a single star. Each of these contracting components can further separate into a double, producing a "double-double" star, the most famous of which is fourth magnitude Epsilon Lyrae. Even more complicated multiples exist. The theory easily explains why doubles are so common.
If the two stars of a pair are fairly close together, and if the plane of the orbit is close to the line of sight, each star can get in the way of the other every orbital turn, and we see a pair of eclipses, one of which is usually of much greater visibility than the other. Eclipsing systems are very important in stellar astronomy, and are used to help determine masses, to find the stars' diameters, temperatures, and even to assess shapes in the cases that the stars' mutual gravities distort each other. Eclipsing doubles are quite common, the most famous second magnitude Algol in Perseus.
In a double star system in which the two have significantly different masses (by far the most common), the higher mass star will use its internal hydrogen fuel the fastest and become a giant first. We then see a red giant, or maybe a helium-fusing, orange class K giant coupled with a main sequence star, also very common. Eventually, the giant produces its planetary nebula and dies as a white dwarf. Good examples of such systems are Sirius and Procyon, each of which are orbited by the tiny dead stars. For each of these systems, and for many others, the white dwarf is by far the LESS massive of the pair, proving that stars really do lose a great deal of their mass back into interstellar space.
If the two stars of a double are close together, they can interact. When the more massive becomes a giant, its surface significantly approaches that of the other star. The lower-mass main sequence star can then raise tides in the giant, distorting it. If the two are close enough, matter can flow from the giant to the main sequence star. Good examples that display such behavior are Algol and Sheliak. In more extreme cases, the lost matter can encompass both stars, creating a "common envelope." Friction will then bring the stars even closer together, making the process go yet faster. The stirring of the lost mass can create unusually distorted planetary nebulae. At the end, the white dwarf created from the giant finds itself very close to the remaining main sequence star. In high mass double stars, the higher-mass component can explode and produce a nearby neutron star or even a black hole companion.
Some giant stars have the masses and internal constructions that allow them to bring by-products of deep nuclear fusion to the stars' surfaces, in the most extreme examples creating carbon stars. Mass lost from one of these enriched giants to a close companion can contaminate the companion with the giant's newly- formed chemical elements. When the giant becomes a white dwarf we are left with a seemingly single star (main sequence or evolved giant) with an odd chemical composition. Only with determined observation can we tell that a dim white dwarf is present. Among the most prominent examples are "barium stars," giants that have very strong absorptions -- and great overabundances -- of the heavy element barium among several others. All seem to be companions of what were once mightier stars that had become carbon stars and that are now reduced to white dwarfs.
If the white dwarf and main sequence remnant of a close double are close enough, the white dwarf can raise tides in the main sequence star, and mass will flow the other way, from the main sequence star to the white dwarf. Theory and observation both show that the flowing matter first enters a disk around the white dwarf from which it falls onto the white dwarf's surface. Instabilities in the disk can make such a star "flicker" over periods of days and weeks, even producing sudden outbursts of light. The star that became the white dwarf had lost almost all of its hydrogen envelope during its own evolution. When enough fresh hydrogen from the main sequence star has fallen onto the white dwarf, it can, in the nuclear sense, ignite, fusing suddenly and explosively to helium. The surface of the white dwarf blasts into space, the star becoming temporarily vastly brighter. On Earth we see a "new" star or "nova" (meaning "new in Latin) erupt into the nighttime sky, not a new star at all but an old one undergoing eruption. Novae are common, 25 or so going off in the Galaxy every year, once a generation one close enough to reach first magnitude. Nova Cygni in 1975 rivalled Deneb, giving the celestial Swan two tails.
In a massive double star system, the more massive of the pair may develop an iron core and explode as a supernova, becoming either a neutron star or a black hole. Either of these stellar remains in turn may raise tides in the more-normal companion, causing matter to flow into a disk around the collapsed body, from which it falls into an immense gravitational field. Matter in the disk is so hot it can radiate X-rays. From the motion of the normal star, we can calculate information on the mass of the collapsed one. If the mass is great enough, we can infer the existence of an orbiting black hole, the best actual proof we have. Fresh hydrogen falling from the disk onto a neutron star can become compressed, fuse to helium, and then explode violently as the helium fuses to carbon, the result an X-ray burst similar in nature to a nova.
The term "supernova" is derived from "nova" in that the supernova is vastly brighter, no matter that the mechanism of the core collapse of a supergiant is completely different from the mechanism of nova production. White dwarfs, however, can also produce supernovae. No white dwarf can exceed a mass of 1.4 times that of the Sun, a limit discovered in the 1930s by Subramanyan Chandrasekhar when he applied relativity theory to the gases in white dwarfs. If the limit is exceeded, even the white dwarf's enormous pressure cannot hold gravity back and the white dwarf must collapse into a neutron star or a black hole or perhaps even annihilate itself. There are two alternative theories for such an event. A massive white dwarf may accept enough mass from a close main sequence companion and be pushed over the edge before a nova eruption can take place. The white dwarf then collapses, creating a supernova that is grander even than one produced by the collapse of a supergiant's iron core. The main sequence star of a double that contains a white dwarf can also evolve through the giant stage to become a white dwarf, creating a DOUBLE white dwarf system. If the two have been drawn close enough together by interaction during a common envelope phase, they can spiral together by the radiation of gravitational waves predicted by relativity theory. The white dwarfs then merge, again producing a spectacular supernova. In either case, the collapse and resulting explosion makes nuclear reactions that create a vast amount of iron and other elements. Kepler's supernova of 1604, the last seen in this Galaxy, was probably of this kind.
Stars can range in size, depending on mass and age, from only a few
kilometers across to the diameter of the orbit of perhaps Saturn.
They can range in temperature from near "cold" at only 2000 K for
an extreme red giant through far over 100,000 K for the star inside
a planetary nebula to over a million K for a neutron star. All the
stars you see in the sky will eventually expire, some soon, some
not for aeons. Lower mass stars create planetary nebulae and white
dwarfs, while higher mass stars make supernovae that result in
neutron stars or black holes. Double stars add spice to the
product, making novae and a different kind of supernova. All these
end products send newly made chemical elements into the
interstellar stew, out of which new stars are made, some perhaps to
be orbited by "earths" that are made from the deaths of earlier
generations of stars, our Sun someday to make its own contribution,
however modest, to generations yet unborn.
